A plurality of embodiments are described in the present application. These embodiments relate to a new and useful improvement in the production of duplex films where trapped printing is desired or necessary. One of the present embodiments will sometimes be referred to in the present application as a supply roll embodiment; this embodiment relates to a supply roll of a two-ply or duplex film which is treated on both outside surfaces for printing or laminating and then wound onto a single supply roll. An additional embodiment described in the present application is sometimes referred to as a printing or trapped printing process; this embodiment comprises a process compatible with producing trapped printing on conventional, unmodified printing equipment without the necessity of adhesives or adhesive inks and is applicable to linking two polyethylene films without the use of adhesives or adhesive inks; it was previously thought impossible to successfully link one layer of polyethylene film to another layer of polyethylene film without the use of adhesives or adhesive inks.
The present invention is based on the discovery that two-ply film treated for printing or laminating on both outside surfaces could be produced on conventional film manufacturing equipment and then wound onto a single storage roll for successful later use. This discovery by itself was so contrary to conventional wisdom that the applicant had considerable difficulty in obtaining such material from film suppliers, who finally provided the material without warranting that it could usefully unwound from the storage roll.
The present printing invention is also based on the discovery that, when two-ply film is manufactured on conventional manufacturing equipment, is treated on both outside surfaces for printing or laminating, and is then wound on a single storage roll, an excellent surface attraction is created when the two treated surfaces are later brought together during an otherwise conventional printing and rewind process.
In carrying out the present supply roll invention, two-ply film that has been treated on both outside surfaces for printing or laminating is wound on a single supply roll. The present printing invention may then be carried out by unwinding the film from the single supply roll as a single two-ply sheet with its outside surfaces treated and by passing the two-ply sheet through a conventional printing process. Printing inks without adhesive properties are preferably used, although adhesive inks and adhesives may be employed if desired. Either a face or reverse image may be used. The film is then rewound, preferably with little or no cooling from conventional drying ovens, onto a single product roll. The outer most ply of the product roll is then preferably unwound for one turn and cut off so that the ends are even, and the product roll is unwound to make packaging or similar products with the ink trapped between the two treated surfaces, which are preferably in intimate face-to-face contact, although water or water vapor may also be introduced for an even greater linking of the surfaces, without adhesive or adhesives being required.
At this point in the preferred process, the two layers of film are linked to each other as a result of the attraction caused by the film treatment, by the heat from the press ovens, and possibly by the apparent discovery that the attraction achieved through the preferred printing process is increased by contact of surfaces that were manufactured at nearly the same time during the film manufacturing process; this latter explanation of the phenomenon by which the present printing invention works is not well understood by the applicant, and may not be correct, but it is postulated as one of the reasons for the success of the present printing process.
As previously indicated, an additional discovery related to the present printing invention is that, if water or water vapor is added to the duplex film layer after printing just prior to rewind, there is a greater attraction between the films.
The linked-duplex film fabricated through the present printing process is useful in the manufacture of premade bags for handfilling as well as with form-fill-seal equipment when using automatic filling equipment. If a film laminate is desired, the only additional requirement beyond that of the preferred process is the application of an adhesive or an adhesive ink during the film converting process. The remaining process is same as in the preferred process.
In U.S. Pat. Nos. 2,579,968 and 2,679,969, Richter describes the use of films that will facilitate a lamination process where the films being laminated have a natural affinity for one another and tend to cohere without adhesives. In U.S. Pat. No. 2,679,969 (see col. 3, lines 32-45), Richter states that a number of films and film combinations do not have the necessary inherent cohesive characteristics required to produce packages formed of laminates of such films and that polyethylene to polyethylene is a structure that falls within this category. However, contrary to this earlier teaching, the present invention enables fabrication of polyethylene to polyethylene linked duplex film structures without the use of adhesives or adhesive inks; further, such structures can now be produced on standard printing equipment without modification.
In U.S. Pat. Nos. 3,462,329 and 3,607,519, Beyer discloses trapped printing and lamination systems requiring two separate unwind or supply rolls. These patents disclose a film lamination using adhesive inks or overall adhesive coatings during the printing or laminating process.
In U.S. Pat. No. 3,462,329 at col. 1, lines 57-61, Beyer notes that, in the process of Richter, it was impossible to use polyethylene films due to the fact that the polyethylene is completely lacking in surface attraction. In col. 2 of this patent, after noting that his invention is compatible with polyethylene, Beyer discloses at lines 54-61 that his process produces a two-ply film laminate in which the films adhere to each other through the adhesive properties of the ink. Further, in col. 3, lines 63-67 of this patent, Beyer also recommends that, if the printing legend is small, an all-over lacquer or adhesive be added for sufficient adhesion.
As previously indicated, the processes disclosed in Beyer's patents require a printing press or other equipment having double unwind stands for separate supply rolls of film. Accordingly, the processes of Beyer cannot be used with conventional printing equipment or with conventional turret unwind stands used for continuous printing so that roll changes do not stop the printing process. When two separate rolls of film are used as in the Beyer processes, they will not normally have the same footage on both rolls, even if the film gauges are the same; this is because the process of film manufacture normally results in film gauge variations, and this in part results in film rolls having varying lengths of film rolled onto them. Accordingly, double roll processes cause more press shutdowns for roll changes due to variations in film footage on the separate rolls.
Further, the processes disclosed in Beyer's patents makes the use of thin films very difficult since for such films the unwind process of two separate supply rolls requires extra or very precise tension controls. For example, as will be recognized by those skilled in the art, use of polyethylene film with thicknesses under 0.001 inch is not presently a preferred manufacturing process where two separate supply rolls are used, due to the stretch and wrinkling that will occur before the two separate film layers come into contact. By way of contrast, the present printing invention can easily be run with two 0.0005 inch polyethylene films.
In U.S. Pat. No. 3,462,329 (see col. 4, lines 6-12), Beyer indicates that much thinner film laminates are possible with his process since the printing is carried out on two plies of film rather than on one thin ply. What Beyer fails to point out, however, is that his process requires that the film be unwound in single ply form, not just from one source but from two separate rolls, before the film is combined into a two-ply sheet for printing. Therefore, in order to control film stretch in very thin films during the Beyer process, there must be extra or unusually precise tension control devices for controlling the film stretch before the two single plies come together to form the two-ply film prior to printing.
In contrast to these prior art processes, the present trapped printing invention allows all conventional film printing presses, including those with conventional unwind stands, single or turret, to produce a two-ply film with locked-in printing. Accordingly, the present printing invention does not require modification of standard printing presses so that double unwind stands are available for creating a duplex or a laminated film. Further, the present printing process eliminates extra roll changes due to film variations, since both layers of film in the present printing process are already on a single unwind or supply roll.
As previously indicated, because film processed through the present printing invention is in the two-ply state from start to finish, the invention allows very thin films to be processed without extra tension controls applicable to systems in which individual thin films are combined after being wound off of separate rolls.
As also previously indicated, the present printing invention allows a linked duplex film to be processed without the use of adhesive inks or adhesives; significantly, this is true whether the size of the printed legend is large or small. This then eliminates the need for additional printing plates and for special inks and adhesives as used in the Beyer process and, accordingly, expense and toxic emissions are reduced.
The present printing process also permits the use of water or water vapor to create greater linking of the two plies, also without special inks or adhesives; this optional feature of the present printing invention is cost effective and does not create emission problems. (Although in U.S. Pat. No. 3,901,755 Martin discloses a method comprising water-induced bonding of films, the method disclosed requires a special chemical treatment of the films prior to the water application; no such special treatments are required if water or water vapor is used to augment the present printing invention.)
The present invention also makes it possible for standard bag-making equipment to make a duplex bag without special unwind equipment or special sealing equipment, since the two layers of the duplex film produced through the present printing process will remain linked together at the bag opening and during the bag-making process.
It is also possible with the preferred process to create a film which is more soft than films produced through conventional lamination processes; this is because there is less material between the layers and an absence of adhesives.
Although the present printing process is particularly applicable to converting and printing polyethylene film, the process is also applicable to polypropylene, coextrusion, or other films that can be plied during film processing.
Further, if the use of adhesive inks or adhesives is desired, the present printing process permits processing through use of adhesive inks or adhesives without any modification of conventional converting equipment; such laminates may be prepared on standard converting equipment by the simple addition of adhesive inks or adhesives within the otherwise identical process of the preferred printing invention. This gives film converters a great deal of versatility and allows many film converters to gain access to these production processes without a major investment in special equipment comprising double unwind stands or other equipment.
As will be further discussed below, the preferred process of the present printing invention uses a supply roll of two-ply film with both outside surfaces of the double ply treated in the conventional manner for printing. In addition, however, the present process may also be used with two-ply film with only one side treated for printing or adhesives, the treated surface being printed, the remainder of the process being as described below; although this latter process is possible, the linking of the duplex films without adhesives or adhesive inks through this approach is substantially inferior to the linking provided using the preferred printing process.
The processes of Richter are based on films having inherent cohesive characteristics. As previously indicated, Richter noted that polyethylene was one material which did not have such a characteristic. In contrast, Beyer's processes are based on the application of adhesive-type inks or adhesives to hold the two layers together. The present printing process eliminates the necessity for two-roll supply systems required of the Beyer processes, eliminates the necessity of adhesives and adhesive inks, and works well with all films capable of being plied, including polyethylene to polyethylene.